This invention relates to fuel cell assemblies and, in particular, to fuel cell assemblies using internal reforming.
So-called direct carbonate fuel cell assemblies are known in which the assemblies convert a hydrocarbon fuel directly to direct current electricity. In these fuel cell assemblies, the fuel is internally reformed to produce hydrogen for fuel cell use. Fuel cell produced water and heat are used by the reforming reactions. The reforming and fuel cell reactions are carried out in thermal and mass transfer communication such that the heat and water produced by the fuel cell are made available in-situ for the reforming reactions.
Two different types of direct fuel cell assemblies have emerged. In the direct internal reforming assembly described in U.S. Pat. No. 3,488,226, reforming catalyst is placed in the anode compartment of each fuel cell directly in communication with the cell environment. In the alternate indirect internal reforming assembly described in U.S. Pat. No. 4,877,693, the bulk of the reforming is carried out internally in reforming units placed in between fuel cell groups in a stack. A group of cells typically might be comprised of five to fifteen fuel cells. The reforming units are in thermal communication with the fuel cells so that the reforming reaction is able to draw the necessary heat from the nearby cells.
A hybrid assembly incorporating both the direct and indirect internal reforming derives the benefits of the direct arrangement (that is high fuel conversion) and the indirect arrangement (longer catalyst life, higher fuel cell performance, and more uniform temperature distribution). In the hybrid assembly, reforming catalysts are placed in the anode compartment of each fuel cell and a reforming plate is placed in between fuel cell groups.
U.S. Pat. No. 5,175,062 describes a reforming unit used for indirect internal reforming and its integration with a direct fuel cell assembly which incorporates a combination of direct and indirect internal reforming. The assembly of the '062 patent has certain disadvantages in reliability and costs which are associated with the fuel supply line connections to the reforming unit.
More particularly, fuel and steam need to be distributed to each of the reforming units. A full-size fuel cell assembly requires 30 to 40 connections for this distribution. In the '062 patent, these connections are made by welding a feed tube to each reforming unit at one end and to a fuel delivery header at the other end. Since the reforming units are electrically live, they must be electrically isolated from the metallic fuel supply header. The '062 patent utilizes dielectric breaks in the feed lines for this purpose.
As can be appreciated, the fuel feed lines of the '062 patent are at the highest pressure with respect to other gas streams. Because of their location external to the fuel cell stack, the entire lines, including their connections at the reforming units and at the dielectric breaks, need to be fully protected against gas leaks for the entire life of the fuel cell assembly. This is made more difficult by the fact that the each feed line connection is subjected to thermo-mechanical stresses during transportation and operation. Therefore, the design of the gas leak protection for the feed lines needs to be very robust to enhance reliability.
The currently available dielectric breaks for voltage isolation of the feed lines comprise ceramic tubes joined with metallic transition pieces, by a brazing process, for ease of connection with the metallic tubes of the feed lines. These types of fittings are expensive and, furthermore, are not rated for the desired &gt;500.degree. C. temperature operation of a carbonate fuel cell assembly, as the brazing compound available for joining the metal feed tube and the ceramic fitting is not stable at these temperatures. Thus, complete gas tightness of these joints throughout the life of the fuel cell assembly is not fully assured.
It is, therefore, an object of the present invention to provide a fuel cell assembly with internal reforming and with improved gas leak protection for the fuel feed lines.